Our aims are to conclusively demonstrate that energy expenditure due to physical activity is critical in the prevention of weight gain & to measure the amount of physical activity needed to maintain weight. By taking the difference between total energy expense from doubly-labeled water & resting metabolic rate + thermic effect of a meal we can obtain energy expended in physical activity in free-living subjects. Thus we can for the 1st time quantitate aspects of lifestyle in the subject's home environment.